Alkoxyl radical reactions with oxygen

The antioxidant role of NO comes Ifom its reaction with oxygen, carbon and nitrogen centred radicals and can be seen to have a scavenger role under a range of conditions [112]. This is because of the unpaired electrons of NO which react rapidly with alkoxyl and alkyl hydroperoxyl radicals at near diffusion reaction rates (2x10 M s [113]). It is these reactions that have been suggested to have a modulatory role in enzyme- or metal-catalysed... 

Ab initio theoretical considerations indicate that the ground electronic state is essentially the carbon-centered radical, and the first excited electronic state is the oxygen-centered radical (Dupuis et al., 1982). Thus the measured rate coefficient is probably for the carbon-centered radical, and need not correlate with information on alkoxyl radical reactions. 

McGarvey, J.J., and W.D. McGrath (1964), Photochemical production of alkyl and alkoxyl radicals and their reactions with oxygen, Trans. Faraday Soc., 60, 2196-2206. 

As described, other nucleophilic reactions in the anthraquinone series also involve the production of anion-radicals. These reactions are as follows Hydroxylation of 9,10-anthraquinone-2-sulfonic acid (Fomin and Gurdzhiyan 1978) hydroxylation, alkoxylation, and cyanation in the homoaromatic ring of 9,10-anthraquinone condensed with 2,1,5-oxadiazole ring at positions 1 and 2 (Gorelik and Puchkova 1969). These studies suggest that one-electron reduction of quinone proceeds in parallel to the main nucleophilic reaction. The concentration of anthraquinone-2-sulfonate anion-radicals, for example, becomes independent of the duration time of the reaction with an alkali hydroxide, and the total yield of the anion-radicals does not exceed 10%. Inhibitors (oxygen, potassium ferricyanide) prevent formation of anion-radicals, and the yield of 2-hydroxyanthraquinone even increases somewhat. In this case, the anion-radical pathway is not the main one. The same conclusion is made in the case of oxadiazoloanthraquinone. 

In a second type of side reaction, the carbanion formed can be alkylated by reaction with the sulphonium ion to yield unexpected products [59], This reaction sequence is followed for the reduction of ethoxysulphonium salts 10. Cleavage of the sulphur-oxygen bond, which is the weakest bond in the species, leads to an alkoxyl radical. This is reduced further to the alkoxide, which attacks an ethoxysulphonium ion to form diethyl ether [60],... 

The definition of an antioxidant suggests a functional assay of antioxidants by measuring inhibition of appropriate (easy to study) oxidation reactions. Such assays can be called inhibition assays for antioxidants (Fig. 2). Various oxidants are used in TAC assays. In many cases, thermal decomposition of 2,2 -azobis (2-amidopropane) (ABAP) is the source of oxidizing radicals. ABAP undergoes temperature-dependent homolysis. The primary radicals produced by thermal decomposition of the initiator react with oxygen to produce peroxyl and alkoxyl radicals, which are oxidizing species in the system (Fig. 3). The amount of free radicals formed in an aqueous medium by decomposition of ABAP at pH 7.4 and at 37°C equals 1.36 x 10-6 [ABAP] x t, where t is time in seconds and [ABAP] is in mol L-1 (N8). 

Eq. 4.54 shows the reaction of n-heptanol (151) with Pb(OAc)4 under high-pressured carbon monoxide with an autoclave to generate the corresponding 8-lactone (152). This reaction proceeds through the formation of an oxygen-centered radical by the reaction of alcohol (151) with Pb(OAc)4,1,5-H shift, reaction with carbon monoxide to form an acyl radical, oxidation of the acyl radical with Pb(OAc)4, and finally, polar cyclization to provide 8-lactone [142-146]. This reaction can be used for primary and secondary alcohols, while (3-cleavage reaction of the formed alkoxyl radicals derived from tertiary alcohols occurs. 

Besides scavenging peroxyl radicals, tocopherols and tocotrienols are also excellent quenchers for singlet oxygen and peroxynitrile (39). It has been reported that ot-tocopherol shows a pro-oxidant activity when present at high concentrations (106-108). It was later found that tocopherols do not show a pro-oxidant effect if they are compared with controls devoid of them and other lipid-related antioxidants (109, 110). However, tocopherols lose their antioxidant efficacy at high concentrations because they participate in a number of reactions that lead to production of alkoxyl radicals that consume tocopherols and initiate new reaction chains because of their low selectivity (111, 112). Using a kinetic approach, a number of reactions were found to contribute to the loss of tocopherol antioxidant activity that is greater for a-tocopherol than for y-tocopherol (113). 

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